Acoustic transducers using piezoelectric elements as an oscillator are known in the literature. For example, U.S. Pat. Nos. 3,832,580 and 3,792,204 describe transducers using a single piezoelectric film; an article by Tamura et al. presented in 1978 at the Acoustical Society Meeting in Honolulu describes a pair of piezoelectric films mounted over the upper and lower surfaces of a polyurethane-foam cushion, and U.S. Pat. No. 3,832,580 describes the use of a plurality of piezoelectric elements suspended in various configurations.
For a given alternating-current voltage, a piezoelectric-film transducer typically generates a lower acoustic amplitude than that produced at the same voltage by other types of transducers, such as electrodynamic transducers. This lower voltage sensitivity can lead to an undesirably low amplitude for certain applications, such as telephone receivers, where the signal voltage is low. Furthermore, the piezoelectric film transducers used as microphones or transmitters generate a lower output voltage for a given sound pressure than do other types of transducers, such as electret condensers. Such low output voltages necessitate the use of high-gain amplifiers, which are often undesirable.
The acoustic transducer disclosed in U.S. Pat. No. 4,295,010, which is hereby incorporated by reference, goes a long way toward improving the output of such piezoelectric transducers. It discloses the use of a plurality of piezoelectric films that are mounted and spaced apart at their peripheries and physically connected near their centers by a dot of epoxy adhesive.